The recovery of phosphorus from wastewater has gathered strength due to its acceptance as a sustainable method for solving wastewater treatment plant struvite problems and the low global reserves of phosphorus ore. Although the chemistry and successful operation of phosphorus recovery plants are well documented, there still exists opportunity to reduce and optimize the use of external resources, such as magnesium, that is required for the nutrient recovery. One of the primary operational costs arises from the need for external magnesium addition, and therefore, the proper (and timely) detection of the element is necessary. In this study, methods were tested which could provide information, on-site, on the rate of application of the element and its concentration in the various water matrices. A method was developed that utilizes the combined use of pH and conductivity to determine the amount of external magnesium that needs to be added to a water sample. The amount required was determined by locating a transition point in the pH-conductivity-external magnesium added graph and the phosphate concentration in the water. For each mole of phosphorus removed, the molar ratio of Mg:P was 1.3-2.0 at the transition point. The magnesium concentration in the water matrix was also determined by the hardness test method; this method was found to be suitable for quick, on-site testing.